Isolate VulkanHppGenerator and VideoHppGenerator in CMakeLists.txt

When adding the vulkan-hpp CMake project (e.g. through FetchContent), people may sometimes want to merely access the headers under `vulkan/` and therefore only need access to the interface library providing the correct include directory:



```cmake

FetchContent_Declare(vulkan-hpp

    GIT_REPOSITORY "https://github.com/KhronosGroup/Vulkan-Hpp.git"

    GIT_TAG "v1.4.303"

    GIT_SHALLOW ON

    GIT_SUBMODULES ""

    OVERRIDE_FIND_PACKAGE

    EXCLUDE_FROM_ALL

    SYSTEM)

FetchContent_MakeAvailable(vulkan-hpp)

```



As the submodules are not necessary for this, the VulkanHppGenerator and VideoHppGenerator targets will not be set up properly (even when not used), as the `TINYXML2_SOURCES` are missing, prompting CMake to complain about not knowing what language the target should be in:

https://github.com/KhronosGroup/Vulkan-Hpp/blob/752778463e89c27677aecaf18cf80449c12ad603/CMakeLists.txt#L366-L383



Fetching as shown will result in the following error during configuration:



```

[cmake] CMake Error at build/_deps/vulkan-hpp-src/CMakeLists.txt:367 (add_executable):

[cmake]   Cannot find source file:

[cmake] 

[cmake]     build/_deps/vulkan-hpp-src/tinyxml2/tinyxml2.cpp

[cmake] 

[cmake]   Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm

[cmake]   .ccm .cxxm .c++m .h .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90

[cmake]   .f95 .f03 .hip .ispc

[cmake] 

[cmake] 

[cmake] CMake Error at build/_deps/vulkan-hpp-src/CMakeLists.txt:367 (add_executable):

[cmake]   No SOURCES given to target: VulkanHppGenerator

[cmake] 

[cmake] 

[cmake] CMake Error at build/_deps/vulkan-hpp-src/CMakeLists.txt:374 (add_executable):

[cmake]   No SOURCES given to target: VideoHppGenerator

[cmake] 

[cmake] 

[cmake] CMake Generate step failed.  Build files cannot be regenerated correctly.

```





It would be nice if these two targets could be hidden behind an if condition or some function, making this repo a convenient way to fetch a specific header version, rather than always generate the bindings.

The current solution is to set `SOURCE_SUBDIR "disabled"` to disable the cmake configuration for this project and adding `vulkan-hpp_SOURCE_DIR` as an include directory, which works but is not as nice as having access to an INTERFACE target, especially when wanting to use the c++20 module

[]